In machine vision systems, one or more cameras can be used to perform vision system processes on an object or surface within an imaged scene. These processes can include inspection, decoding of symbology, alignment, and/or a variety of other tasks. In some applications, a machine vision system can be used to inspect a flat work piece disposed within or passing through an imaged scene. The scene can be imaged by one or more vision system cameras. The images captured by the cameras can be processed by the machine vision system to determine information about features within the imaged scene, such as features' real-world locations, poses, etc. In some applications, the camera can be calibrated to allow the machine vision system to perform vision tasks with increased accuracy and reliability.
A calibration target can be used to calibrate the cameras. A calibration target can be any object with accurately known (e.g., as a result of the manufacturing process) or measurable dimensional characteristics. For example, a calibration plate can be used as a calibration target. A calibration plate can be a flat object with a distinctive pattern made visible on its surface. The distinctive pattern can be laid out such that the machine vision system or user can easily identify visible features in an image of the calibration plate acquired by a camera. Some exemplary patterns include, but are not limited to, dot grids, line grids, a honeycomb pattern, a checkerboard of squares, a checkerboard of triangles, etc. Calibration patterns typically contain highly localizable features. The level of accuracy in localization of features can affect the accuracy of calibration. Circles (dots or holes) can be less accurate than grid or checkerboard patterns. Characteristics of each visible feature are known from the plate's design, such as the position and/or orientation relative to a reference position and/or coordinate system implicitly defined within the design. The features present in the design can include calibration features. Calibration features can be individual features in a calibration plate's design whose characteristics can be readily measured (for example, the feature's position, size, color, etc.), and whose intended characteristics by the nominal design are known. For example, some possible calibration feature positions include the corners or centers of checkerboard-pattern tiles, or the centers of dots in a dot grid, or the line crossings in a line grid. Calibration plate designs can include a large number of calibration features arranged in a repetitive pattern. More basically, a calibration feature can be readily extracted from an acquired image and can provide known characteristics. In some applications, the known dimensional characteristics are used to calibrate the machine vision system.
In some conventional systems, calibration targets have certain characteristics. For example, calibration targets are typically dimensionally stable and rigid. Calibration plates can be made from a variety of materials. In some applications, calibration plates can be made from glass (e.g., by chrome-on-glass fabrication). Glass calibration plates have characteristics that can be beneficial for machine vision system calibration. For example, the dimensions of a glass calibration plate do not significantly vary with changes in temperature and humidity (e.g., dimensionally stable). However, glass calibration plates are typically expensive. In some applications, calibration plates can be made from biaxially-oriented polyethylene terephthalate (BoPET), such as MYLAR polyester film produced by Dupont. While BoPET calibration plates typically cost less than glass calibration plates, the dimensions of BoPET calibration plates can vary with changes in temperature and/or humidity (e.g., by more than 1/10th of a percent). Similarly, some metal calibration plates (e.g., calibration targets made from aluminum or steel plates) are not dimensionally stable (e.g., such calibration plates can suffer from significant thermal expansion). Paper and BoPET calibration plates are not dimensionally stable. Additionally, paper and BoPET calibration plates are not rigid, unless being affixed to a rigid supporting substrate.